This invention relates to a hub cap assembly for a wheel bearing. The invention is particularly, but not solely, directed to a hub cap for a wheel bearing on a boat trailer.
Hub caps are normally placed on wheel bearings to prevent ingress of dirt, dust, water and other contaminants to the bearing, and thereby prolong the life of the wheel bearing. The hub cap also serves to retain lubricating grease in the bearing cavity.
Wheel bearing assemblies on trailers are lubricated by hand packing grease into the bearings at the time of assembly. Thereafter, wheel bearings, particularly those on trailers, are seldom checked (especially as trailers do not normally undergo regular servicing). Hence, the loss of lubricating grease from the wheel bearing may go unnoticed until the bearing fails. (With loss of lubrication, the bearing runs extremely hot, particularly at highway speeds. The bearing materials are likely to fail at high temperatures, with possibly catastrophic results if the vehicle is travelling at high speed).
In most wheel assemblies, there is little, if any, provision for the permanent lubrication of the inboard or rear seal of the bearing. Consequently, many rear seals will dry up and/or burn out in a short period of time in the absence of lubrication, thereby creating opportunity for water to enter the bearing.
It is known to use sealed hub caps on wheel bearings to prevent loss of lubrication. However, the interior of a wheel bearing is subject to a wide range of temperatures. From ambient temperature when stationary, the temperature inside the wheel bearing can increase quite quickly to over 90xc2x0 C. when the vehicle is travelling at highway speeds. With such increase in temperature, the resultant pressure increase within the sealed bearing can cause the inboard grease seal to blow out, resulting in loss of lubrication.
Vented hub caps are sometimes used to accommodate temperature increases in the bearing chamber. By venting the hub cap to the atmosphere, the pressure within the bearing chamber is maintained at or near atmospheric pressure.
Although vented hub caps are able to handle pressure changes due to variations in the temperature of the bearing, they give rise to particular problems when subjected to rapid temperature changes. For example, wheel bearings on boat trailers are usually at an elevated temperature after a road trip to a boat launching ramp. When the hot wheel bearings are submerged in the water during the boat launching procedure, the temperature of the bearing chamber is reduced rapidly, leading to a sudden pressure drop or partial vacuum in the bearing chamber. As a result, water and other contaminants are sucked in through the hub cap vents and/or the inboard seal.
Due to the substantially closed configuration of the bearing chamber, such water can remain in the bearing for a considerable period of time. This leads to rust, particular if the bearing is unused for extended periods as is common for boat trailers. The rust problem is exacerbated if the water is salt water.
It is an object of the present invention to provide an improved hub cap for a wheel bearing which overcomes or ameliorates at least some of the above described disadvantages, or which at least provides the consumer with a useful choice.
The invention provides a hub cap for a wheel bearing. The hub cap is in the form of a cup-like housing adapted to be fitted to a wheel hub containing the bearing. The cup-like housing preferably has at least one transparent portion to enable the level of lubrication in the housing to be ascertained visually.
Typically, the cup-like housing is of cylindrical form, and the transparent portion is a circular glass end wall sealingly fitted to the cylindrical housing.
Advantageously, the hub cap is a sealed or closed hub cap, rather than a vented hub cap. The closed construction of the hub cap prevents ingress of contaminants to the wheel housing.
More preferably, the hub cap includes at least one flexible portion adapted to accommodate pressure changes within the hub cap housing.
In the preferred embodiment, the hub cap includes a pressure equalisation device having a membrane sealingly located in an aperture in the cup-like housing. The inner side of the membrane is subject to pressure within the housing, while the outer side of the membrane is vented to the atmosphere. The flexible membrane can deform to substantially equalise the pressure within the hub cap housing with the outside pressure, while maintaining the sealed nature of the hub cap.
A filler plug or similar device is suitable provided on the housing to allow lubricating material to be fed to the wheel bearing via the hub cap.
Preferably the lubricating material is oil. The oil not only serves to lubricate the bearings, but also protects the rear seal from drying out. Since the rear seal is permanently lubricated by the oil, it remains waterproof. As the front hub cap is also waterproof, the complete wheel bearing is sealed, and suitable for marine applications.
The hub cap is particularly suitable for trailers, such as boat trailers.
In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings.